Adsorbed gas storage provides an advancement in the temporary storage of gaseous materials that overcome obstacles related to high pressure systems commercially used today to compress gases.
Gases such as noble gases, O2, N2, hydrocarbons, and other small gas molecules are used in many markets including, but not limited to, industrial, automotive, pharmaceutical, food, beverage, electronics, etc. Specific applications include concentrated medical oxygen, industrial desiccants, portable and residential liquefied petroleum gas (LPG), stranded gas flares, kerosene burners, cook stoves, compressed natural gas (CNG), and many others.
The storage and transport of said gases are typically in vessels where high pressures are used to condense or even liquefy the gas to utilize the greatest concentration safely possible with in an occupied volume. The pressurization process results in unwanted and excessive costs due to the energy and infrastructure requirements. In addition, there is a limitation in storage containment geometry to safely store and transport the gas; resulting in limitations of storage designs. The use of sorbents to store gases at lower pressures is known to solve some of these issues.
The technology to convert low bulk density sorbent powder into an immobilized higher density block using a thermoplastic binder is well known for filtration applications. It is also widely believed that high surface area sorption materials formed into high density compacted structures can achieve the economic storage volume needed for gases.
U.S. Pat. No. 7,708,815 describes the unique needs and challenges for the storage of hydrogen, achieved by a hydriding/dehydriding process in the presence of non-porous metal compounds, which are in the form of a fine powder of less than 30 microns. A thermoplastic binder is used to minimize packing of the metal compounds during hydriding/dehydriding cycles. The patent fails to address the needs and challenges to bind other active particles, such as carbons and molecular sieve sorbent, which have significant porosity (>40%), and larger particle sizes (>100 microns), especially those with a BET surface area greater than 1,400 m2/g. Porosity is defined as the ratio of void space in a volume to the total volume. Surprisingly in these cases, we found that the role of the binder is not to minimize packing of active particles, but actually to maximize such packing. The patent also fails to teach an appropriate range for the binder particle sizes to allow for maximizing packing and good mechanical integrity of the composite hydrogen storage material, as the binder particle size of <0.1 microns is too small to effectively bind activated carbon and molecular sieve particles.
U.S. Pat. No. 4,999,330 describes the needs and challenges for the high-density sorbent used in ANG systems. High surface area activated carbon is the sorbent typically used in high density sorbent structures. However loose carbon particles have the drawbacks of low packing density, and the ability to move sorbent with the gas stream and cause potential contamination.
The U.S. Pat. No. 4,999,330 reference solves the above limitations by forming a methyl cellulose or polyvinyl alcohol binder solvent solution, coating high surface area carbon particles with the binder solution, followed by removing the solvent and compressing the binder-coated particles to cause a bulk volume reduction of 50 to 200%. The '330 system suffers from its complexity and many steps. It also involves coating the entire activated carbon particles with polymer solution—which ultimately blocks many of the micropores—this fouling reducing the amount of surface area available for adsorption. Some of the pores can be pre-filled with solvent which can later be removed by heat to unblock many of the pores, however the net effect of a full coating is a large reduction in active surface area.
US2017/0007982 uses a water soluble cellulosic binder to coat active carbon particles and form a monolith. The monolith also suffers from a complicated manufacturing process, and significant fouling of the activated carbon pores due to the binder coating. In addition, the monolith requires the incorporation of a scaffold material, to achieve good mechanical integrity. Scaffold materials include natural or synthetic fibers, such as polyester and polypropylene fibers.
Carbon block targeted for filtration applications are described in U.S. Pat. Nos. 5,019,311, 5,147,722, and 5,331,037, uses an extrusion process to produce a porous article containing bound active particles, such as activated carbon, bound together by a thermoplastic binder. The carbon block filter is designed to remove contaminants from a fluid stream—such as in the purification of water. The polymer binder, which is generally a polyethylene, is needed at a high level.
U.S. Pat. No. 6,395,190 describes carbon filters and a method for making them having a 15 to 25 weight percent of a thermoplastic binder. The problem with polyethylene and other typical binders is that high loading percentages are required to adequately hold the sorbent materials together. Poor fouling resistance is also a problem
Poly(vinylidene fluoride) as a binder for carbon block filters, has been found to improve the carbon block article performance by providing effective binding at lower loading—which in turn provides greater efficiency by reducing the pressure drop when the fluid passes through the block.
Examples of such carbon block filters, as well as methods for producing them, are described for example in WO 2014/055473 and WO 2014/182861, the entire disclosures of each of which are incorporated herein by reference for all purposes. These articles use polyvinylidene fluoride or polyamide binders, rather than the polyethylene binders previously used for carbon block filtration articles. There is no mention of the use of such systems for small molecules storage, or with sorbent of high BET specific surface area greater than 1,000 m2/g.
In the area of gas storage, there is a need to improve the volume of gas that can be stored in a given volume of container space (v/vo), to improve the economics of this technology. The activated carbons used in filtration systems have high ball hardness and moderate BET surface area, in order to minimize the pressure drop when a fluid passes through the system. In contrast, activated carbons with low ball hardness and high BET surface areas are needed to effectively store gases, and pose different challenges when it comes to combine them with a binder. In this case, the binder needs to maximize packing of the carbon particles, instead of minimizing it in the case of filtration systems.
Surprisingly it has now been found that a high v/vo can be obtained in a gas storage article using low levels of polyvinylidene fluoride, polytetrafluoroethylene, or polyamide binders of moderate particle sizes, with suitable activated carbon. The low levels of binder have little negative effect on the ratio of activated sorbent volume to container volume. Moderate binder particle size is needed for efficient packing of the activated carbon particles, if the binder particles are too large (>1 micron), they allow for some carbon particles to remain loose, if the binder particles are too small (<0.1 micron) they can't reach multiple carbon particles which lead to ineffective binding. In addition to providing a solid adsorption article having a high density packing, low level of binder volume, the solid porous sorbent structure of the invention also show excellent resistance to fouling of the sorbent. In some cases, fouling is further reduced by processing the article of the invention at temperature equal or below the melting point of the binder. Lastly, polyvinylidene fluoride, polytetrafluoroethylene or polyamide binders also provide excellent chemical resistance to the gas storage environment. Additionally, the high relative thermal index of these polymers is useful for the temperature range an adsorptive storage monolith would encounter during the product life cycle.